Abstract
Damage to non-regenerative neurons in the central nervous system (CNS) is often irreversible, leading to lasting cognitive deficits or even death. During CNS infection, the immune system walks a tight line controlling pathogen replication and preventing excessive inflammation and neuronal death, as seen during Toxoplasma gondii (T. gondii) infection. T. gondii is an intracellular parasite that disseminates systemically during the acute phase of infection before establishing a chronic infection in the brain. In the CNS, T. gondii lies dormant in a cyst form and spontaneously reactivates, resulting in damage and cell death. Subsequent peripheral immune cell infiltration is critical to restrict parasite replication and promote survival. Human Single Nucleotide Polymorphisms (SNPs) affecting inflammatory cell death form pyroptosis have been associated with CNS pathology in response to T. gondii CNS infection, indicating that pyroptosis plays a vital role in determining the outcome of CNS infection. Pyroptosis releases preformed immunogenic molecules, or alarmins, that recruit immune cells and enhance parasite control in the infected brain. Caspase-1 is the major enzyme mediating pyroptosis, yet whether caspase-1 is necessary for protective immune responses against T. gondii is unclear. This is largely due to the late generation of mice specifically deficient in caspase-1. Historically, pyroptosis has been studied primarily in vitro and attributed mainly to monocytes and macrophages. It is unclear which cell types undergo pyroptosis during T. gondii and whether distinct cell types' pyroptosis will result in distinct impacts on immune responses to the parasite. Previous work suggested that microglia, the CNS resident macrophage, underwent pyroptosis to promote immune control of T. gondii, but a lack of tools that efficiently and specifically genetically target microglia hindered this hypothesis from being tested.
Here, we demonstrate that during acute T. gondii infection, both tissue-resident macrophages and a non-macrophage population utilize caspase-1 to exert distinct effects on the immune response. A macrophage population releases IL-18 to reinforce adaptive immune activation against intracellular pathogens, whereas the non-macrophage population likely releases IL-1 cytokines and the alarmin S100A11 to recruit neutrophils and monocytes, respectively, to the site of infection. Furthermore, using IL-1 receptor accessory protein knockout mice, we show that IL-1 receptor signaling and neutrophil recruitment are dispensable for acute T. gondii control. This work identified the concerted effort of multiple cell populations, caspase-1, contributing to distinct aspects of the immune response to T. gondii.
In the CNS, we found that caspase-1 promotes monocyte recruitment and parasite control. To distinguish the roles of microglia and peripheral macrophages, we developed a generalizable strategy to enhance the specificity of Cx3cr1CreERT2 for microglia. This approach leverages macrophage ontogeny and transient CSF1R inhibition: pharmacologic depletion of macrophages followed by withdrawal of inhibitor allows peripheral niches to be repopulated by untargeted bone marrow–derived progenitors, while microglia renew locally, thereby achieving microglia-specific recombination. We validated that this approach does not alter host immune responses to T. gondii, indicating that differences in peripheral macrophage ontogeny are unlikely to affect parasite control. Using this approach, we identified that peripheral macrophages, rather than microglia, utilize caspase-1 to promote CNS immune responses during T. gondii infection. Overall, the work presented in this thesis provides conceptual insight into the multifaceted roles of the inflammasome during T. gondii infection and establishes an approach to enhance the specificity of genetic targeting in microglia.
